The terms green tea, oolong tea and black tea are used to put tea into rough categories, so I am going to define them very basically in this post.
NB In each picture, the cup on the left contains new dry tea leaves, in the centre is the tea itself, and on the right are the same tea leaves after brewing tea (though a smaller quantity as all tea leaves enlarge on brewing).
Green, oolong and black refer to different levels of oxidation. Green tea is not oxidised, oolong tea is partially oxidised and black tea is completely oxidised.
The analogy that I always use (though I didn’t make it up) is that if an apple is cut open, it will begin oxidising and so it will turn brown. The longer the apple is left, the browner it will become. This is basically what happens with oolong and black tea. The leaves are slightly bruised and allowed to oxidise, and so turn browner. Black tea is allowed to completely oxidise, and so the leaves turn black, hence the name. In Chinese, black tea is called red tea ( 紅茶 hóng tsá), because the resulting tea liquid is reddish in colour.
Oolong can be lightly or heavily oxidised, depending for how long they are processed. Oxidation is brought to an end by heating the leaves until they reach around 65 degrees celcius (150 Fahrenheit). This is done in China by pan-firing the tea, and in Taiwan, our farmers use oven-like machines or heated tumbling machines (they look a little bit like cement mixers or sort of rotating grills) to control the oxidation. Oolong in Chinese means black dragon (烏龍), as the first oolongs,and other kinds of oolongs than the one below, are black twisted leaves similar in appearence to the black tea above, and therefore look like dragons…
Green teas are heated shortly after picking and drying, so that they retain their colour. We can go back to the apple analogy. If apples are cut and then put into an apple pie, they will retain their original white colour because of the heat, and this is what happens with green tea, and why the leaves are still green. In Chinese, green tea, is just green tea “lìu tsá 綠茶”, and both the leaves and the tea are green.